Metal/air electrochemical cells, such as zinc/air cells, and batteries constructed with such cells, exhibit several advantageous characteristics suggesting their potential use in lighting applications in which relatively low operating voltages are required for extended periods of time. Air cathodes, which employ an essentially unlimited reactant source (the atmosphere) are not consumed during discharge and are of low weight. Additionally, metal/air systems exhibit high discharge capacities.
To date, however, the metal/air system has been limited to military batteries and button cells, i.e., watch, calculator and hearing-aid cells. Past attempts to fabricate a metal/air cell for other applications have failed due to the lack of a cell design which was leak resistent, stable towards environmental effects, and yet of adequate rate capability for other applications, such as typical lantern drains. Additionally, prior metal/air cells have not been capable of accommodating the growth of anodic material during discharge, while at the same time being inexpensive enough to manufacture and capable of sufficient design flexibility to be commercially viable.
The chief problems and limitations associated with metal/air battery systems stem at least in part from the same factor which provides for their advantages: interaction with the environment. For example, the loss or gain of water in the cell as a result of environmental humidity changes has been a significant design consideration. Additionally, a metal/air cell must be designed to accommodate the increase in the volume of anodic material that occurs during discharge. For instance, in a zinc/air cell, the oxidation of zinc to zinc oxide during cell discharge can cause an increase in volume of about 17%. The internal pressure created by this increase in cell volume often causes such cells to leak. Additionally, this expansion of anodic material introduces the risk of short circuit by contact between the positive and negative electrodes.
It is known in the art to provide compressible means within the cell to absorb expansion of the anodic material during discharge. Such an arrangement is for instance described by Jammet, U.S. Pat. No. 3,855,000. This teaching, however, exacerbates one known disadvantage--a larger and more expensive cell design is required to accommodate the compressible means. Additionally, inclusion of the compressible means imposes certain design constraints which limit the applicable uses of the cell.
It has also been proposed to provide free space inside the cell in order to permit expansion of the anodic material during discharge. However, when the individual anode particles, which are often initially suspended in a gelled alkaline solution, expand freely into the void space, they may not remain in physical contact with one another, resulting in incomplete cell discharge. Additionally, if the cell is not sealed tightly, the anode may be directly oxidized by oxygen from the atmosphere. As a result, the performance capacity of the cell will be reduced. It has also been suggested that the discharge of the cell may be effected by shifting of the active anodic material within the free space.
Furthermore, if a large amount of free space is required in order to permit adequate expansion, a larger, more expensive battery must be constructed. For instance, because the cell casings of zinc/air button cells are conventionally constructed of rigid, metallic materials, which are not capable of expanding to accomodate the growth of anodic material during discharge, such cells typically require an initial void space of 17%.
Accordingly, it is an object of the present invention to provide a cell case for use in manufacturing a metal/air electrochemical cell which is suitable for consumer and industrial lighting applications, is capable of accomodating the expansion of anodic material during discharge, possesses a reduced amount of void space, and produces more power than traditional cells.
It is another object to provide a cell case for use in manufacturing a metal/air electrochemical cell which is leak resistant, stable towards environmental effects, and has an adequate rate capability for typical lantern drains.
It is another object to provide a cell case for use in manufacturing a metal/air electrochemical cell which has a smaller uncomplicated structure, is inexpensive to manufacture, and is easily fabricated in a wide variety of designs suitable for diverse applications.
It is another object to provide a metal/air electrochemical cell and battery manufactured with such a case.
These objects and other subsidiary objects which will be apparent to those skilled in the art are achieved by the practice of the present invention.